This application is based on and claims priority from German Patent Application No. 198 01 390.6 filed Jan. 16, 1998, which is incorporated herein by reference.
The invention involves equipment and a procedure for the suppression of whole echoes comprised of several partial echoes in telecommunications (TC) equipment, such as end devices, transmission systems or switching equipment, with the help of adaptive FIR (=Finite Impulse Response) filters, which reproduce the whole echo and extract it from the useful signal impeded by the echo, which has been transmitted to the respective TC equipment.
Such a procedure and equipment for adaptive echo compensation is known from the German patent DE 44 30 189 A1.
With the transfer of speech signals via telecommunication lines, acoustic echoes can originate either on the xe2x80x9cnear endxe2x80x9d from the user sending out the signal due to direct sound transfer from the loudspeaker to the microphone of the end device. The xe2x80x9cnear echoxe2x80x9d problem is considerably intensified when several end devices are set up closely together, for example in an office or a conference room with multiple telephone connections since there is a coupling from each loudspeaker signal to each microphone. The multi-channel echo suppression procedure and the associated electronic circuit proposed in the EP 0 627 825 A2 should help in this case. In addition to the near acoustic echo, it is possible that a composite electric line echo is also created from a great variety of partial echoes due to the reflections of the speech signals sent out to positions which are varying distances away in the transmission channel, for example in the case of 2-4 wire hybrids (hybrid circuits) in the end device or in the exchanges and at network interconnections. In this case it is usually so-called xe2x80x9cshort distance echoesxe2x80x9d which are reflected back to the speaker in a time period up to 128 ms. These short distant echoes are differentiated from long distance echoes, which come back to the speaker in a timeframe of up to 640 ms and are especially disturbing due to the large time delay since in the meantime the speaker is already considerably advanced in his speech by the time his own reflected speech signal reaches him again. Such delayed long distance echoes occur for example during intercontinental calls, or during long distance telephone calls transmitted via deep-sea cables or satellites. The network operators which maintain such telephone networks are therefore especially interested in suppressing long distance echoes with large time delays, but also short distance echoes reflected back with higher intensities as effectively as possible.
To this end, for example in EP 0 792 029 A2, echo suppression equipment with an adaptive filter is proposed, including a coarse and a fine detector for near end speech signals which are set up on both sides of the echo suppression equipment and are consequently able to monitor the incoming signal before and after the echo suppression.
An adjustment to the reflected echo value should be carried out in speech pauses on the near end.
DE 44 30 189 A1 quoted at the beginning proposes a cost-efficient procedure which can be used under various acoustic conditions, and which uses a FIR filter whose filter coefficients are determined using the NLMS algorithms. According to this method, echo suppression in a TC network with multiple parallel channels utilizes an adaptive FIR filter for each channel, which is realized in software on a digital signal processor. The adaptive FIR filter is supposed to reproduce the expected whole echo, which may be composed of several individual partial echoes and extract it from the useful signal which has been transmitted to the TC end device and is impeded by the echo.
In contrast, the purpose of the present invention is to develop the procedure and equipment of the type indicated at the beginning to the point of enabling the effective suppression of echoes, without a large amount of storage space and computational effort, even for a large number of m parallel TC channels which are independent of each other, for example as they are in exchanges.
In accordance with the present invention, this task is solved in that for m parallel, independent TC channels to be served, n homogeneous FIR filters or partial filters, each of which can reproduce a whole or partial echo, are implemented on a single ASIC (=Application Specific Integrated Circuit), whereby m, n xcex5 N and especially nxe2x89xa72; that the whole or partial echoes to be reproduced can be calculated in real time on the ASIC; and that there are provisions for a digital signal processor (=DSP) which controls the ASIC and can carry out the filter settings in the ASIC, especially after the coefficients and time-delays required to reproduce the echo have been calculated in the DSP.
Through the high integration density of an ASIC, the equipment in accordance with the present invention can be kept especially compact. Since the digital signal processor is quite considerably relieved through the integration of the FIR filter on the ASIC, many more parallel TC channels can be served with constant processor effort with effective echo suppression. This allows for the processing of larger time delays in the echoes (for example up to 640 ms), multiple individual echoes for each whole echo and many parallel channels ( greater than 2000) with minimal effort.
More advantageously, a number of FIR partial filters are cascaded in order to reproduce a whole echo. At present the demands of TC network operators call for three to five partial echoes to be suppressed per channel. However, this could still increase in the future.
It is especially preferred that the equipment in accordance with this invention be designed in such a way that a variable subset of FIR partial filters can be assigned to a TC channel to be served, as required, depending on the number of partial echoes as well as the size of the delay times. Through this flexible assignment of FIR filters per processed channel according to the number of individual echoes and according to the size of the delay times between the signal and echo, the ASIC can be of an extremely compact design despite the very high number of channels.
An advantageous advanced set-up distinguishes itself in that there are provisions for a macrocell for each FIR partial filter in the ASIC. This macrocell includes a shift register of suitable length for the temporary storage of the sampled speech signals, demultiplexer equipment for setting the effective shift register length, as well as adding equipment for the summation of the sample value weighted with the appropriate coefficients. These macrocells can be precisely the same construction, which simplifies the manufacture of an ASIC with many homogeneous FIR partial filters and therefore makes it less expensive.
It is especially easy to have a cascading of several macrocells that enables any choice of flexible interconnections, for example for reproducing a whole echo, and therefore an especially high capacity utilization ratio of the macrocells.
A preferred advanced set-up provides for a macrocell including one of the coefficient memories approximated by the digital signal processor, which can be directly read by a modulo N counter as well as a digital multiplier unit with an operating frequency of fA=Nxc2x7fS, to which the appropriate coefficients from the coefficient memory and the appropriate sample values from the shift register are fed for multiplication, in which case the sample values with a sampling frequency of fS have been stored in the shift register. There are provisions for an adder which adds the product values output by the digital multiplier unit to each of the sums of the preceding product values stored in an initial temporary memory via a feedback loop and which can finally be extracted from the useful signal impeded by an echo, which has been stored in a second temporary memory as the reproduced partial echo signal and transmitted to the TC equipment. This type of construction only requires a single multiplier, which replaces an appropriate number of multipliers in the conventional FIR partial filters for the processing of N. The operating frequency fA increased by the factor N in comparison to the operating frequency fS is no problem with the standard electronic component parts available today, provided that N moves in the magnitude of 50-100.
One especially effective form of the equipment in accordance with the present invention provides for several coefficient memories, which can all be read by means of a demultiplexer, whereby each of the TC channels to be served can be assigned appropriate coefficient memory, and in which the suppression of echoes on all TC channels can take place via a single FIR whole filter consisting of several FIR partial filters, which can be switched to the multiplex mode.
Preferred is another design in which there are provisions for a storage device, preferably a RAM (=Random Access Memory), in which the current sampled speech signals which have been transmitted from the TC equipment can be stored, as well as the coefficients belonging to the reproduced partial echoes, and in which there are provisions for a control unit, preferably integrated into the ASIC, which administers the process of echo suppression by means of an address calculation of data stored in the storage unit, the DSP and the FIR partial filters on the ASIC. In this way, the coefficient memories and sample value memory can be taken from storage in the ASIC and packed on an extremely inexpensive RAM which is available in high density. The shift register is also eliminated in the design described above.
The control unit essentially controls the process of the data flow, so that very little hardware is necessary for realizing the echo suppression equipment and the controlling digital signal processor is relieved to a large degree, since it essentially only has to carry out the calculation of the coefficients, preferably for several independent TC channels.
Especially preferred is an advanced design of this equipment in which the time demultiplex procedure m serves independent TC channels. In this form, the address calculation in the control unit replaces the multiplex units from the designs described further above.
In another preferred design of the equipment in accordance with the present invention, there are also provisions for equipment in the recognition of non-speech signals on a TC channel, especially for the detection of modem or fax signals. In this case, the echo suppression function can be turned off automatically for non-speech signals, which increases the lack of distortion of fax and modem connections.
Another advanced design of this form provides for equipment in the recognition of non-speech signals in which a discrete Fourier transformation (=DFT), especially a Goertzel algorithm, is implemented. This requires especially little storage area and computational capacity for this additional function.
Moreover, in another advanced design of this form, there is a compander circuit for masking line echoes. This makes the echo suppression even more effective, and therefore comfort for the end user as well.
The compander circuit can also be realized on an ASIC, preferably on the same ASIC as the FIR filter. The hardware units are therefore especially compact and inexpensive to produce.
Within the framework of the present invention, there is also a process for the operation of echo suppression equipment in accordance with the present invention in which a Dirac impulse is sent to the corresponding TC channel, on which the echo answer is detected and the suitable coefficients for reproducing the whole echo are calculated and stored for the corresponding FIR filter.
In other process variants, defined analog noise signal sequence is sent to the appropriate TC channel instead of the Dirac impulse, the echo answer on it is detected and the suitable coefficients for reproducing the whole echo are calculated and stored. The duration of sending out the noise signal sequence is normally less than a second.
Particularly preferred is an advanced design of this process variation in which the analog noise signal-sequence includes a Gaussian noise signal limited to the bandwidth of the TC channels to be served.
Another alternative process variation is distinguished from other variants in that a synthetic, preferably ternary pseudo-noise sequence is sent to the corresponding TC channel, the echo answer on it is detected, and suitable coefficients for reproducing he whole echo are calculated and stored. The pseudo noise sequence is usually selected in such a way that only the main value of the sequence shows a correlation value q while the secondary values have the correlation value 0.
It is particularly preferred that coefficients for reproducing the echo are gathered from the correlation of the echo signals that have been sent to those which have been received.
Also preferred is an advanced design of the procedure in accordance with this invention in which an NLMS (=Normalized Least Mean Square) algorithm is used for the calculation of the filter coefficients for the calculation of the coefficients from the received echo signals in the knowledge of the signals sent out. In this process, the CPU time and computational effort necessary for attaining a satisfactory result is kept especially small. In this case, the length of the noise sequence is advantageously adjusted to the length of the FIR filter.
In another process variation, the whole echo composed of all the partial echoes to be considered is put together on the ASIC and is extracted as a whole from the useful signal transmitted to the TC equipment. In this case, only one position of the incoming signal on the TC channel to be served is affected.
Alternatively, the partial echoes to be considered can be reproduced in the ASIC and individually extracted from the useful signal transmitted to the TC equipment. In this way, the design of the hardware and computer units in accordance with the design of the present invention is kept flexible.
Additional advantages of the invention can be seen from the description and sketches. The features mentioned above and described further in accordance with this invention can also be used separately or in any desired combination. The designs shown and described here are not to be understood as a complete list, but rather as exemplary features as an illustration of the invention.